Page 101 - Kaleidoscope Academic Conference Proceedings 2020

Industry-driven digital transformation

• Upgrading Wireless TN: In addition, network operators splitting) options range from tens to hundreds of µs. When
also need to upgrade their wireless TN. In doing so, more functions reside in DUs than RUs, C-RAN is more
frequency spectrum, bandwidth, and the corresponding beneficial, but heavy centralization creates more burden for
wireless technologies affect link capacity, range (distance) the front-haul network (FN), and there is a need to balance
and other practical considerations. functional splitting between RAN and FN. Upgrading FN is
A 10 Gbps wireless link may be adequate for some time consuming, and it is vital to decide early on C-RAN
networks [15], but cannot be provided in sub 40 GHz scenarios with priority given to dense urban areas [24].
bands. In order to provide sufficient capacity, ETSI has As stated above, it is beneficial to deploy C-RAN in 4G
proposed to use mmWave bands, including V-band and networks, which also facilitates deployment of centralized
E-band which can provide up to 10 GHz bandwidth for 5G NR. However, CPRI interface in 4G networks is
wireless links [20]. Studies are also underway to propose TDM-based, which is inconsistent with packet-based 5G
higher bands, including W-band and D-band. fronthaul interfaces. To resolve this, the IEEE 1914 Working
Another challenge is the provision of quality of service Group has developed radio over Ethernet (RoE) standard for
(QoS) in 5G for different services and a range of QoS encapsulation of digitized radio over Ethernet frames. RoE
requirements. However, wireless links are time-varying includes a CPRI mapper that maps/de-maps CPRI frames
and have diverse propagation characteristics that impact into/from Ethernet frames. By utilizing RoE CPRI mapper,
their QoS. Hence, different frequency bands are used an Ethernet-based 4G C-RAN can be deployed and used for
for different services, depending on QoS requirements. 5G C-RAN. In brief, centralization can begin from hotspots
ETSI has introduced band and carrier aggregation (BCA) in 4G, and then 5G NR base stations can be added when the
technology to collect information from the media and map capacity and latency requirements of fronthaul interfaces are
each service to a proper band. BCA can extend the range satisfactory.
of wireless links by sacrificing availability [21].
5.3 Software-Defined Transport Networks
• Expanding fiber-based TN: Gigabit passive optical Software-defined networking brings many benefits such as
networks (GPONs) are already deployed by many network flexibility, scalability, and agility for TNs as specified in
operators, but in some cases, cannot fulfill both capacity the transport SDN (T-SDN), and can also be used for
and latency requirements of 5G. NG-PON2 is a promising deploying efficient TN slicing. Network slices are realized
candidate for replacing GPONs, but may not be available by coordinating the TN management system and E2E
soon. As an alternative, standardization of 50 Gbps/100 management system. A TN management system can be
Gbps PON is underway. Ethernet interfaces should also implemented via the SDN controller [17]. To evolve a legacy
be upgraded to increase capacity. Although GbE may be TN to T-SDN, TN devices and TN architecture should both
sufficient for eNBs, they are inadequate for gNBs. Hence, be upgraded. In what follows, we describe the upgrading
GbE interfaces must be upgraded to 10 GE/25 GE for of TN devices (i.e., forwarding/routing devices) and TN
fronthaul interfaces. In brief, to increase TN capacity, architecture.
existing links should be upgraded first, to be followed by
the deployment of new links. 5.3.1 Upgrading Transport Network Devices
Migration Working Group of Open Networking Foundation
5.2 C-RAN Deployment and Transport Networks (ONF) has categorized three types of devices for 5G TN
C-RAN is an efficient concept for optimal allocation of deployment:
resources in cellular networks, but TN is very challenging in • Legacy devices: devices with integrated CP and DP;
C-RANs. In 4G, the common public radio interface (CPRI) • SDN-enabled devices: devices in which CP is decoupled
is used, whose data rate linearly increases with bandwidth from DP, and CP is external to the device; and
and number of antennas. CPRI is not suitable for 5G with • Hybrid devices: devices with both legacy DP and CP with
more bandwidth and massive MIMO technology. Hence, SDN capabilities. Many legacy devices are upgradable to
enhanced CPRI (eCPRI) is introduced for TN in 5G, which be SDN-enabled via software/hardware upgrades.
exploits functional splitting to reduce latency and data rate There are three approaches for migrating from legacy TNs to
requirements compared to CPRI. Moreover, eCPRI utilizes SDN-enabled TNs:
cost-effective packet-based technologies such as Ethernet and • Greenfield deployment [25]: upgrading existing TN
IP, where F1 interface, also called mid-haul, is used to connect devices to the SDN-enabled version.
DUs to CUs, and NG interface connects CUs to 5G CN. • Mixed deployment: co-existence of SDN-enabled devices
The data rate and latency of interfaces are impacted by with legacy devices, where the SDN controller and legacy
3GPP functional splitting options. Option 2 (between radio devices need to exchange routing information via legacy
link control and packet data convergence protocol layers) CP agents; and
covers DU-CU functional splitting [17] (also called higher • Hybrid deployment: co-existence of legacy, hybrid and
layer splitting), with latency in the order of ms [22, 23], SDN-enabled devices, where hybrid devices are upgraded
while data rate is reduced to near the end-user data rate. to become SDN-enabled, and legacy devices are gradually
Latency for DU-RU functional splitting (called lower layer replaced by SDN-enabled devices.

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